The invention relates to carrier hetrodyning and reference frequency generation techniques to achieve accurate carrier frequency generation in Optical Frequency Division Multiplexing (OFDM) systems.
FIG. 1 shows the general configuration of an OFDM (Optical Frequency Division Multiplexing) system, and FIG. 2 discloses a prior art frequency tracking system used in each transceiver 2-1, 2-2, etc. of an OFDM system.
In FIG. 1, numerals 2-1, 2-2 . . . 2-N, and 2-i, designate a group of transceivers each coupled by two-way optical fibers 35 to a passive coupler 31 that sometimes is referred to as a "star coupler". Numeral 2-M designates a transceiver containing a Reference Frequency Generator that produces a single reference frequency f.sub.M on optical fiber 54, which is used as the sole reference frequency for the entire OFDM system. Each transceiver, including 2-i, includes a laser 2 that transmits an optical signal back to star coupler 31, from which that optical signal is received by all of the other transceivers coupled to coupler 31. The frequency of the transmitted carrier signal produced by laser 2 is controlled by a frequency tracking system 6 that receives the various optical signals from coupler 31. A receiver circuit 28 of each transceiver receives those optical signals from coupler 31 via two-way optical fibers 35. An information data input signal V.sub.IN controls laser 2 to produce digital information by modulating the transmitted carrier signal produced by laser 2 so it produces bursts of light that represent logical "ones". In the OFDM system of FIG. 1, each "node" or transceiver sends out an optical signal by tunable laser 2 at a frequency different from the others. Passive coupler 31 combines or multiplexes all of the signals in the optical domain. This arrangement makes all of the optical signals visible to all of the receivers, which can tune in any channel by using a tunable filter for incoherent detection or a tunable laser to generate a local carrier for coherent detection.
The graph of FIG. 1A shows the spectrum of the carrier signals produced by each of transceivers 2-1, 2-2, etc. The frequency tracking system 6 in each transceiver "references" the transmitted carrier signal produced by that transceiver to the single reference frequency f.sub.M. The spectrum of signals shown in FIG. 1A is applied to the receiver 28 and tracking circuit 6 of each transceiver in the system.
In an OFDM system as in FIG. 1, it is highly desirable that the channel separation be as close as possible, so that many channels can be provided in the system. Obviously, the channel separation .DELTA.f cannot be very small unless each of the frequency spectrums f.sub.1, f.sub.2 . . . f.sub.N is very stable, that is, if those frequencies do not change appreciably as a function of time, temperature, or other factors.
Referring to FIG. 2, passive coupler 31 of FIG. 1 is shown and the lasers in each of the transceivers 2-1, 2-2, . . . 2-N also are shown and are designated by numerals 2-1', 2-2', 2-N', respectively. The frequency spectrums of these lasers are indicated by frequency spectrums f.sub.1, f.sub.2, f.sub.N. Passive coupler 31 distributes output optical signals each of which contains all of the frequency spectrum components f.sub.1, f.sub.2, f.sub.N and also the sole reference frequency f.sub.M. Numerals 6A, 6B, and 6C designate prior art frequency tracking systems that are included in the various transceivers.
Block 6A of FIG. 2 shows details of a prior art frequency tracking system. It includes a Fabry-Perot filter 33 that receives from coupler 31 a signal P.sub.in (t) containing all of the frequencies f.sub.1, f.sub.2, . . . f.sub.N and reference frequency f.sub.M. Fabry-Perot filter 33 produces an optical output which is detected by a photodetector 34 and converted to an electrical signal. That electrical signal is input to a first servo loop including mixer 36-1 and low pass filter 35-1, and also to a second servo loop including mixer 36-N and low pass filter 35-N. The output of low pass filter 35-1 is coupled to an input of Fabry-Perot filter 33. The feedback to the control input of Fabry-Perot filter 33 adjusts its resonance bands to align them in the desired relationship to the incoming optical signal P.sub.in (t) carried by optical fiber 7A. This arrangement allows frequencies in the signal P.sub.in (t) at the resonance frequencies of Fabry-Perot filter 33 to pass through Fabry-Perot filter 33 and be detected by a photodiode in optical receiver 34. The second servo loop (including low pass filter 35-N) modulates the frequency of laser N to adjust its frequency so that its transmitted carrier frequency will pass through Fabry-Perot filter 33.
A major problem with the system of FIG. 2 is that each of the frequency tracking circuits has a different Fabry-Perot filter 33, and those different filters are likely to have substantial differences in frequency spectrums and pass band separation. As a practical matter, this prevents the channel separation from being as small as desired, and therefore greatly reduces the number of communications channels that can be provided in the system of FIG. 1.
The above technique uses the reference frequency f.sub.M, from which each carrier frequency f.sub.1, f.sub.2, etc. generated by the lasers 2-1', 2-2', etc. is kept at a corresponding constant frequency distance, respectively, and maintains the frequency difference in the "optical domain", by means of Fabry-Perot filter 33.
OFDM systems are different from so-called Wavelength Division Multiplexing (WDM) systems in that the channel separation for OFDM systems is much smaller, typically of the order of 0.1 nanometers in wavelength. Compared to the wavelengths of 1300 nanometers or 1500 nanometers commonly used in transmission in OFDM systems, this channel separation is extremely close and requires the wavelength accuracy of a practical useful tunable laser to be maintained within one to ten parts per million. Thus, precise frequency control is crucial for OFDM systems.
All existing methods of reference frequency generation for OFDM systems use only the one reference frequency f.sub.M shown in FIG. 1A for the other carriers f.sub.1, f.sub.2, etc. to follow. The main existing technique for generating accurate carrier frequencies is to use the one reference frequency for all other frequencies to "lock to" in the optical domain. Typically, a single Fabry-Perot filter is used for frequency locking. A Fabry-Perot filter has equal pass band separation, due to Fabry-Perot resonances. The filter outputs are used to indicate whether the laser frequencies are locked or not. That is, if a laser output frequency coincides with one of the pass bands of the Fabry-Perot filter, the optical receiver will detect a signal, which means that the frequency is locked. On the other hand, if the laser frequency has drifted outside the pass band, there is no signal going to the optical receiver. As a result, the Fabry-Perot filter drift determines the OFDM spectrum and channel separation.
A good single frequency laser has wavelength drift of approximately 0.09 nanometers per degree Centigrade, or sixty parts per million per degree Centigrade. A typical Fabry-Perot laser is worse, with drift of about 330 parts per million per degree Centigrade. To make HDWDM or OFDM useable, very precise frequency control is essential, and frequency drift due to temperature variations and other causes must be kept within very narrow ranges.
There are known stabilization techniques for tuning circuits of coherent hetrodyne receivers in which an input signal is mixed with a local oscillator signal and the result is detected by a photodiode. The output signal produced by the photodiode is amplified and sent to an intermediate frequency (IF) filter. The output of the IF filter is applied to the input of a detector circuit and also is applied by means of a servo loop to the local oscillator to stabilize the local oscillator frequency.
There is an unmet need for a technique for greatly increasing the data throughput of an OFDM system, and more particularly, there is a need for technique for greatly decreasing the channel separation therein.